1. Technical Field
The present invention generally relates to variable gain amplifiers, and more particularly to a variable gain amplifier with a wide gain range and a linear-in-dB characteristic for changing an external control voltage.
2. Background Art
An ultrasound diagnostic device has become an important and popular diagnostic tool due to its non-invasive and non-destructive nature. Modern high-performance ultrasound imaging diagnostic devices and techniques are commonly used to produce two- or three-dimensional images of internal features of patients.
The ultrasound diagnostic device generally uses a probe containing an array of piezoelectric elements to transmit and receive ultrasound signals. The ultrasound diagnostic device forms an image of human internal tissues by electrically exciting transducer elements to generate ultrasound signals that travel into the body. Echoes reflected from tissues and organs return to the transducer element and are converted into analog electrical receive signals (hereinafter referred to as “analog receive signals”). The transducer elements may output low amplitudes of the analog receive signals. Thus, the amplitudes of the analog electrical receive signals must be pre-amplified. The pre-amplification is carried out by a pre-amplifier installed on an output terminal of the transducer elements.
When the ultrasound signals are propagated into the tissues of the target object, their amplitudes are attenuated. Thus, the attenuation of the ultrasound signals has to be compensated to obtain an accurate ultrasound image. The compensation may be achieved by adjusting gain of the pre-amplified analog receive signals. The gain of the analog receive signals is usually adjusted by a variable gain amplifier.
The conventional variable gain amplifier may adjust the gain of the analog receive signals by using a passive element having a predetermined absolute value. In such a case, the absolute value of the element may vary according to an operation voltage or operation temperature. Thus, it may be difficult to accurately adjust the gain of the analog receive signals.
The compensated analog receive signals are inputted into an analog-to-digital converter (ADC) for digital processing. That is, the analog receive signals are converted into digital receive signals by the ADC. Subsequently, receive-focusing and digital signal processing are carried out upon the digital receive signals to thereby form ultrasound image data.
The analog receive signals may be amplified by the variable gain amplifier beyond an amplitude range allowable for input into the ADC. If the analog receive signals are amplified beyond an allowable amplitude range as an ADC input or a recovery time is increased due to overload, then the ADC may malfunction. As such, an accurate ultrasound image signal may not be obtained. Thus, a wide-band variable gain amplifier capable of outputting amplified analog receive signals within a limited amplitude range is needed.